Optical discs must have sufficient mechanical properties to ensure reliable writing and reading operation. Optical discs using injection molded resin disc substrates, however, are difficult to fabricate to consistent mechanical properties. It is often necessary to measure mechanical properties of such discs.
Among mechanical properties of optical discs, the axial runout of an optical disc during rotation is generally measured using an instrument having an optical head. Since focus control is made such that an objective built in the optical head may follow the disc surface, this instrument determines an axial runout and an axial acceleration by measuring the travel distance of the objective. The travel of the object can be determined from a change of the capacitance between a stationary electrode and a movable electrode attached to the objective (see JP-B 7-52512). The instrument utilizing this principle is marketed, for example, as optical disc mechanical precision instrument model ODA-II by Shin-Denshi Kogyo K. K. and optical disc mechanical property instrument LM series by Ono Sokki K. K.
The above-described prior art instruments carry out measurement by causing the objective of the optical head to follow the axial runout of a disc during rotation, determining the travel of the objective, and calculating an axial runout or axial acceleration from the travel. This procedure ensures a high precision of measurement, but takes a long time of the order of minute from the start of measurement to the end of calculation. If a whole number of discs are to be inspected in the manufacturing line, a plurality of instruments must be installed to compensate for a drop of productivity. However, since the instruments are expensive, it is practically very difficult to inspect a whole number of discs.